Serine proteases make up the largest and most extensively studied group of proteolytic enzymes. Their critical roles in physiological processes extend over such diverse areas as blood coagulation, fibrinolysis, complement activation, reproduction, digestion, and the release of physiologically active peptides. Many of these vital processes begin with cleavage of a single peptide bond or a few peptide bonds in precursor protein or peptides. Sequential limited proteolytic reactions or cascades are involved in blood clotting, fibrinolysis, and complement activation. The biological signals to start these cascades can be controlled and amplified as well. Similarly, controlled proteolysis can shut down or inactivate proteins or peptides through single bond cleavages.
While serine proteases are physiologically vital, they also can be hazardous. Their proteolytic action, if uncontrolled, can destroy cells and tissues through degradation of proteins. As a natural safeguard in normal plasma, 10% of the protein matter is composed of protease inhibitors. The major natural plasma inhibitors are specific for serine proteinases. Diseases (associated protease given in the parentheses) such as pulmonary emphysema (cathepsin G), adult respiratory distress syndrome (chymases), and pancreatitis (trypsin, chymotrypsin, and others) are characterized by uncontrolled serine proteases. Other proteases appear to be involved in tumor invasion (plasmin, plasminogen activator), viral transformation, and inflammation (kallikrein). Thus the design and synthesis of specific inhibitors for this class of proteinases could offer major therapeutic benefits.
Thrombus formation, that is blood coagulation, is normally initiated by tissue injury; its normal purpose is to slow or prevent blood loss and facilitate wound healing. There are other conditions, however, not directly connected with tissue injury that may promote the coagulation process and lead instead to harmful consequences; examples of such conditions are atherosclerosis and inflammation.
The complex pathways of blood coagulation involve a series of enzyme reactions in which plasma coagulation factors, actually enzyme precursors or zymogens, are sequentially activated by limited proteolysis. Blood coagulation, or the coagulation cascade, is viewed mechanistically as two pathways, the extrinsic and the intrinsic (FIG. 1). Each pathway proceeds through a sequence of the Roman-numeral-designated factors until they converge at the activation of factor X after merger of the pathways. Thrombin generation proceeds stepwise through a common pathway. Thrombin then acts on the solution plasma protein, fibrinogen, to convert it to stable insoluble fibrin clots, thus completing the coagulation cascade.
The extrinsic pathway is vital to the initiation phase of blood coagulation while the intrinsic pathway provides necessary factors in the maintenance and growth of fibrin. The initiation of the coagulation cascade involves the release of tissue factor (TF) from injured vessel endothelial cells and subendothelium. TF then acts upon factor VII to form the TF/FVIIa complex (where VIIa designates the activated factor rather than the zymogen form). This complex initiates coagulation by activating factors IX and X. The resulting factor Xa forms a prothrombinase complex that activates prothrombin to produce the thrombin that converts fibrinogen to insoluble fibrin. In contrast, the intrinsic system is activated in vivo when certain coagulation proteins contact subendothelial connective tissue. In the sequence that follows, contact factors XII and XI are activated. The resulting factor XIa activates factor IX; then factor IXa activates factor X thereby intersecting with the extrinsic pathway.
With time, the TF/FVIIIa complex (of the extrinsic pathway) loses activity due to the action of tissue factor pathway inhibitor (TFPI), a Kunitz-type protease inhibitor protein which, when complexed with factor Xa, can inhibit the proteolytic activity of TF/FVIIa. If the extrinsic system is inhibited, additional factor Xa is produced through the thrombin-mediated action in the intrinsic pathway. Thrombin, therefore, exerts a dual catalytic role in (a) the conversion of fibrinogen to fibrin and (b) mediating its own production. The autocatalytic aspect of thrombin production affords an important safeguard against excessive blood loss, and, assuming presence of a threshold level of prothrombinase, ensures that the blood coagulation process will go to completion.
While the ability to form blood clots is vital to survival, there are disease states wherein the formation of blood clots within the circulatory system can cause death. When patients are afflicted with such disease states, it is not desirable to completely inhibit the clotting system because life-threatening hemorrhage would follow. Thus, it is highly desirable to develop agents that inhibit coagulation by inhibition of factor VIIa without directly inhibiting thrombin.
Need for the prevention of intravascular blood clots or for anti-coagulant treatment in many clinical situations is well known. Drugs in use today are often not satisfactory. A high percentage of patients who suffer internal injuries or undergo certain surgical procedures develop intravascular blood clots which, if unchecked, cause death. In total hip replacement surgery, for example, it is reported that 50% of the patients develop deep vein thrombosis (DVT). Current approved therapies involve administration of heparin in various forms, but results are not entirely satisfactory; 10-20% of patients suffer DVT and 5-10% have bleeding complications. Along these lines, see International Publication No. WO 00/15658.
Other examples of clinical situations for which better anticoagulants would be of great value are when patients undergo transluminal coronary angioplasty and treatment for myocardial infarction or crescendo angina. The present therapy for these conditions is administration of heparin and aspirin, but this treatment is associated with a 6-8% abrupt vessel closure rate within 24 hours of the procedure. Transfusion therapy due to bleeding complications is required in approximately 7% of cases following the use of heparin. Occurrences of delayed vessel closures are also significant, but administration of heparin after termination of the procedure affords little beneficial effect and can be detrimental.
Heparin and certain derivatives thereof are the most commonly used anti-clotting agents. These substances exert their effects mainly through inactivation of thrombin, which is inactivated 100 times faster than factor Xa. Two other thrombin-specific anticoagulants, hirudin and hirulog, are in clinical trials (as of September 1999). However, bleeding complications are associated with these agents.
In preclinical studies in baboons and dogs, the targeting of enzymes involved in earlier stages of the coagulation cascade, such as factor VIIa or factor Xa, prevents clot formation and does not produce bleeding side effects observed with direct thrombin inhibitors.
Several preclinical studies reveal that inhibition of TF/FVIIa offers the widest window of therapeutic effectiveness and safety with respect to bleeding risk of any anticoagulant approach tested including thrombin, platelet, and factor Xa inhibition.
A specific inhibitor of factor VIIa would provide clinicians with a valuable and needed agent that would be safe and effective in situations where the present drugs of choice, heparin and related sulfated polysaccharides, are no better than marginally effective.
There exists a need for a low molecular weight specific serine protease inhibitors specific toward various enzymes, particularly for factor VIIa that does not cause unwanted side effects. 
The FIGURE illustrates the extrinsic and intrinsic pathways of blood coagulation.